The invention relates to a method and apparatus for monitoring the battery of a hybrid vehicle, in which the charge level of the battery is determined and is indicated to the operator.
A large number of methods are known which are based on determining the charge level of the battery by integration of the battery current and subsequent balancing of the charge quantities extracted and supplied. Balancing is the normal method in all battery types whose internal resistance and whose no-load voltage in the central charge region are not an unambiguous function of the instantaneous charge level. When balancing, the discharging efficiency and the charging efficiency constitute uncertainty factors which depend in a different manner on the currents passed through, battery temperature, aging and further influencing factors, such as memory effects, which are difficult to determine.
The known methods for charge balancing attempt to take into account the various influencing variables in a suitable manner and to eliminate further error sources, such as those resulting from long-term integration, for example. A comparatively well-proven method and further literature references can be found in Sohleuter, W. and others: "Ein Gerat zur Ermittlung des Ladezustandes yon Bleiakkumulatoren" [A device for determining the charge level of lead-acid accumulators]. In: E and M, Year 102, 1982, Issue 2, pages 82-87.
As a result of the influencing factors, which are difficult to take into account, balancing becomes so imprecise with increasing time and increasing energy throughput, that it is impossible to derive any reliable conclusions from it on the charge level of the battery. It is thus necessary to charge the battery fully repeatedly in order that the balancing can be restarted from a reliable limit value. Since full charge can be sensed well during the charging process on the basis of the rise in temperature, pressure and voltage, this reset of the balancing can be initiated automatically, as a function of physical battery parameters.
While, in the case of purely electric vehicles, the system makes it necessary to return to a charging station virtually daily, so that full charging and hence a reset of the balancing can be carried out regularly with an adequate charging time (for example overnight), in the case of hybrid vehicles this procedure would unnecessarily diminish the convenience and utilization of the vehicle. This is because hybrid vehicles, which are driven by an internal combustion engine and/or an electric motor, need to return to a charging station less frequently, or in special cases not at all, depending on operating conditions.
It is thus possible in the case of hybrid vehicles for the most recent reset of the charge balancing to have been carried out so long ago that the charge level indication deviates greatly from the actual charge supply of the battery. In this case, the operator of the vehicle should fully charge the battery, so that the balancing would be restarted.
The object of the present invention is to provide a method of battery monitoring based on charge balancing in such a manner that the special conditions in hybrid vehicles are better taken into account, and unnoticed unreliability of the charge level indication is prevented.
By means of the method according to the invention, the operator of the vehicle is warned in good time if the charge level indication is no longer dependable so that balancing should be reset from a defined limit value. Furthermore, the method according to the invention ensures that this requirement occurs as rarely as possible, and only when urgently necessary.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.